1. Field of the Invention
The present invention relates to a top-emission type organic light emitting apparatus that includes a plurality of organic light emitting devices (organic EL devices) with two or more emission colors.
2. Description of the Related Art
In recent years, attentions have been paid to self-emitting devices for flat panels. The self-emitting devices include plasma-emission display devices, field emission devices, and electroluminescence (EL) devices.
Of those, in particular, the organic light emitting devices have been energetically studied and developed. An area-color type organic light emitting apparatus, such as one with a single color of green or that with further added blue, red, or any of other colors, has been commercialized. Currently, development of a full-color type has been actively conducted.
The organic compound layer of the organic light emitting apparatus, through which emitted light passes, has a thickness almost the same as or less than the emission wavelength. Thus, for obtaining a device having a desired emission color and a good luminous efficiency, it is known that a design in consideration of the influence of optical interference is required.
Japanese Patent Application Laid-Open No. 2000-323277 discloses a technology for improving the luminous efficiency of an organic electroluminescent multicolor display that includes an array arrangement of organic EL devices with different emission colors. In other words, the luminous efficiency of the organic electroluminescent multicolor display is enhanced by providing some functional layers (such as a hole transport layer and an electron transport layer) having the same functions of organic compound material layers except a light emitting layer with different thicknesses corresponding to an emission color.
The general structure of the organic light emitting device includes organic compound layers and an upper reflection electrode with a low work function, which are sequentially formed on a glass substrate. The organic compound layers include a transparent anode made of ITO, a hole transport layer, a light emitting layer, and an electron transport layer. Such device structure described above is generally referred to as of a bottom-emission type; the emitted light is discharged from the rear side of the substrate after transmitting through the anode having optical transparency.
An organic light emitting apparatus having a two-dimensional array of such bottom-emission type organic light emitting devices may be driven by an active matrix system. In this case, however, there is a disadvantage in that the bottom-emission type device has a small opening ratio because of the presence of a thin film transistor (TFT) and wiring on a substrate. For improving such disadvantage described above, a so-called top-emission type organic light emitting device has been proposed. The device is able to discharge emitted light in the laminating direction of organic layers.
In each of Japanese Patent Application Laid-Open No. 2005-276542 and Japanese Patent Application Laid-Open No. 2004-014360, a technology for use of TFT to drive a top-emission type organic light emitting device has been disclosed. In this case, a lower reflection electrode is driven as a cathode, and an upper transparent electrode is driven as an anode. In Japanese Patent Application Laid-Open No. 2005-276542, a technology for use of amorphous TFT to drive the device has been further disclosed.
The top-emission type organic light emitting device requires formation of an optically-transparent electrode (i.e., a transparent or translucent electrode) on an uppermost layer of the organic compound layers on a side opposite to the substrate. For instance, an ITO film is formed on the organic compound layer by sputtering. The organic compound layers in such a case have been known to be damaged by oxidation or the like, which leads to an increase in a drive voltage of the device. For preventing the device from being damaged by such a sputtering process, for example, the formation of an ITO film is carried out by a coating process in Japanese Patent Application Laid-Open No. 2004-014360. In contrast, in Japanese Patent Application Laid-Open No. 2005-276542, a buffer layer is formed on the organic compound layers to eliminate an increase in voltage due to the formation of an ITO film by sputtering.
In any of Japanese Patent Applications Laid-Open Nos. 2004-14360, H10-270171, and H10-270172, an electron transport material and one of an alkali metal, an alkali salt, and an alkali oxide are employed for an electron injection layer to reduce a drive voltage. Japanese Patent Application Laid-Open No. 2005-183265 discloses a technology that enables an electron injection from various kinds of metal electrodes. Such a technology employs a layer made of a mixture of cesium carbonate and an electron transport material, which can be easily handled, as an electron injection layer.
An optical interference should be considered for improving the luminous efficiency of an organic light emitting device. In general, the maximum luminous efficiency can be obtained under the conditions in which the distance between the reflection surface and the emission surface is an uneven multiple of λ/4 with respect to the emission wavelength λ. In other words, the distance between the reflection surface and the emission surface needs to be changed depending on emission wavelength. However, when the thickness of the organic light emitting device is adjusted with typical charge transport materials, the drive voltage may differ according to emission colors due to high resistivity of the charge transport material. Thus, there is a problem in that the organic light emitting device is hard to drive under such conditions. In this case, a driving circuit, wiring, and the like should also be changed depending on emission color. In particular, in the case where the organic light emitting device is driven using a thin film transistor (TFT), it becomes difficult to satisfy the same drive characteristics for every emission color when the drive voltage differs largely. In particular, when the distance between the reflection surface and the emission surface is set to 3λ/4 or more, it is found that the above problem appears markedly due to an increase in thickness of a charge transport layer and an increase in difference between the drive voltages.